Not far from Chicago, in a region now dominated by cornfields and whitetail deer, scientists say they've found fossil evidence of a “shark nursery” where prehistoric predators hatched.

The finding, which challenges long-held notions about ancient marine life, highlights a collection of prized local fossils preserved for more than 300 million years by a rare geologic process and then brought to the surface in recent decades by coal mining.

Michael Coates, a University of Chicago evolutionary biologist, said the Bandringa sharks likely spent most of their adult lives in the rivers that run through present-day Ohio and Pennsylvania, citing fossils found in those states in recent decades. But Coates and a colleague suggest that the long-snouted critters laid their eggs and spent the early part of their lives in shallow coastal waters, such as the sea that once covered Illinois' Mazon Creek area and much of the Midwest.

The Illinois fossils were found in Will County in the 1960s and '70s as strip mining altered the landscape south of Chicago. Coal companies would discard piles of dirt rich with fossils, said Paul Mayer of the Field Museum, and allow people to pick through the churned earth.

Amateur archaeologists and experts alike combed through the piles, and many of the fossils they brought home ended up at the Field Museum, which has the two Illinois samples studied by Coates and his co-author, a public display about Mazon Creek, and thousands of the region's specimens in storage.

Mazon Creek's fossils began forming hundreds of millions of years ago when flowering plants and grass were nonexistent and when dinosaurs — not to mention humans — had yet to roam the Earth, Mayer said.

“When these things died, they fell into the mud of an estuary or even freshwater ponds in a little delta-like area,” said Mayer, who oversees about 40,000 Mazon Creek specimens as the Field's fossil invertebrate collections manager. “They were buried in the mud, and, for whatever reason, iron came in and cemented the rock around them.”

The process preserved many organisms that would have simply decomposed elsewhere, allowing today's scientists to study ancient jellyfish, worms and the soft-bodied Tully Monster. (Found only in Illinois, the aquatic critter is the state fossil.) It also led to a fuller picture of sharks, including the fossils Coates and fellow researcher Lauren Sallan wrote about in an article published online this week in the Journal of Vertebrate Paleontology.

“The preservation of Mazon Creek allowed us to reconstruct this animal in this detail using the fossil record,” said Sallan, now a University of Michigan evolutionary biologist, who started the research five years ago as a graduate student rotating through Coates' Chicago lab.

Sallan and Coates' paper analyzed two Bandringa samples from Mazon Creek originally identified by scientists as separate species. The juvenile sharks, just 4 to 6 inches long, had pronounced spoon-billed snouts that stretched half as long as their bodies and, Coates said, “looks a little bit like the things you see today in sturgeon, paddlefish.” Their findings suggest that the two sets of fossils are in fact members of the same bottom-feeding species and a juvenile version of the adult sharks found fossilized in Ohio and Pennsylvania.

That young Bandringas — but not adults — have turned up in Illinois and adult ones — but not their offspring — were found farther east suggests that the sharks thrived in freshwater but used saltwater havens (like the one south of present-day Chicago) as a “shark nursery” to lay eggs and allow young animals to live safely, Coates and Sallan said.

Although no sharks living today are known to travel from freshwater to saltwater to lay eggs, most sharks do use shark nurseries.

“Almost all sharks and their relatives today use nursery waters,” Sallan said. “They usually use an environment very near the shore because the shallow area protects the juveniles from other sharks that may be too big to enter them.”

The findings, both scientists say, were possible only because of the fossils found south of Chicago, which are renowned in scientific circles even if they're unknown to many locals. Coates said he learned about the fossils as a graduate student in the United Kingdom and was eager to study them when he arrived here.